The present invention relates to a press roll of adjustable sag which cooperates with a mating roll. In particular, the invention is concerned with force producing means at the ends of the press roll shell for deflecting and moving the ends.
A press roll with adjustable sag is known from U.S. Pat. No. 4,213,232 and its equivalent Federal Republic of Germany application No. 28 26 316. Such rolls define a press nip with a mating roll and attempt to produce a high pressing force which is as uniform as possible over the length of the press nip. For instance, when using such a roll in the press section or in the calender of a paper making machine, it is important to remove the water from the web of paper as uniformly as possible over its width and to obtain a thickness of paper web which is as uniform as possible over its width.
It may be necessary to lift the press roll, for instance, upon an interruption in its operation. One advantage of the known roll construction is that the roll shell can be lifted a few centimeters off the mating roll without it being necessary for this purpose to lift the entire press roll, including the supporting yoke. Such lifting might be done, for instance, by arranging the entire press roll on pivoted levers. Preferably, the roll supporting yoke, together with its support pedestals, remains at all times firmly attached to the foundation, machine frame, bearing housing of the mating roll, or the like. The support pedestals may also be part of the machine frame. One merely shifts the roll shell in the press plane, transverse to its longitudinal direction. The mating roll can rest in completely unmovable bearings.
The hydraulic support device in the known press roll, for countering the mating roll in the nip is developed as a number of individual piston elements, which are displaceable radially in the press plane, and are disposed in cylindrical bores in the stationary yoke. The rotating roll shell of the press roll slides over the outer end surfaces of these non-rotating piston elements. When the direction of the force of the piston elements is from the bottom to the top, that is the piston elements are below the top of the roll shell, the press nip can be opened simply by reducing the hydraulic pressure on the roll shell lifting piston elements, so that the roll shell moves away from the mating roll under its own weight.
In U.S. Pat. No. 4,213,232, the placement of the piston elements is reversed. Therefore, the press nip is opened by lifting off the roll shell in opposition to the force of gravity. For this purpose, a hydraulic lift device is provided at both ends of the roll. It is in the form of an auxiliary piston which is arranged for radial displacement in the yoke. When that piston is acted on by pressure, it lifts the guide part on which the roll shell is mounted.
One disadvantage of this construction is that the pistons of the hydraulic support device which transmit the supporting forces from the yoke to the roll shell are arranged exclusively within the axial length of the press zone. Therefore, the so called pressing force profile, that is, the distribution of the pressing force over the length of the press zone, can be influenced only within very narrow limits, namely in the manner that the different pistons of the hydraulic supporting device are acted on by different pressures, a technique known from other publications.
In U.S. Pat. No. 4,414,890, which is equivalent to European Pat. No. 0043119, particularly FIG. 2, a different construction is described. There the roll shell is supported at each end by means of a self aligning bearing on the support pedestal of the yoke. The axial centers of the roll shell bearing and of the bearing of the mating roll lie at least approximately in the same transverse plane. In other words, the distance between the bearings of the roll shell is approximately equal to the distance between the bearings of the mating roll. That patent teaches that this bearing arrangement is important for producing a precisely uniform pressing force profile over the length of the press zone. In particular, a w-profile, which is undesirable in the art, is avoided. Furthermore, in FIG. 2 of U.S. Pat. No. 4,414,890, supplementary hydraulic support pistons are provided between the end of the press zone and the roll shell bearing. These apply additional forces to the ends of the roll shell either in the direction toward the mating roll or in the opposite direction. In this way, one can produce different pressing force profiles. But this construction also does not always afford the possibility of applying a sufficiently high additional force to the ends of the roll shell, among other reasons, because the ends of the roll shell slide over the additional support pistons.
This construction also has the disadvantage that the roll shell is no longer displaceable relative to the yoke along the press plane, that is, toward and away from the mating roll. Therefore, if it is desired to open the press nip, the entire press roll must be lifted off the mating roll by means of pivoted levers or linear guide elements, for instance, in accordance with German Pat. application P 36 10 107.9(see U.S. Pat. No. 4,796,452). Of course, the mating roll could just as well be lifted off the press roll. Pivoted lever arrangements have the disadvantage that they take up a large space and tend to vibrate. The proposed construction with linear guide elements avoids these disadvantages, but there is the unfavorable factor that the yoke must also still be displaced together with the roll shell, as previously.